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diff --git a/include/olm/megolm.h b/include/olm/megolm.h
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+/* Copyright 2016 OpenMarket Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef OLM_MEGOLM_H_
+#define OLM_MEGOLM_H_
+
+/**
+ * implementation of the Megolm multi-part ratchet used in group chats.
+ */
+
+#include <stdint.h>
+#include <stdlib.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * number of bytes in each part of the ratchet; this should be the same as
+ * the length of the hash function used in the HMAC (32 bytes for us, as we
+ * use HMAC-SHA-256)
+ */
+#define MEGOLM_RATCHET_PART_LENGTH 32 /* SHA256_OUTPUT_LENGTH */
+
+/**
+ * number of parts in the ratchet; the advance() implementations rely on
+ * this being 4.
+ */
+#define MEGOLM_RATCHET_PARTS 4
+
+#define MEGOLM_RATCHET_LENGTH (MEGOLM_RATCHET_PARTS * MEGOLM_RATCHET_PART_LENGTH)
+
+typedef struct Megolm {
+    uint8_t data[MEGOLM_RATCHET_PARTS][MEGOLM_RATCHET_PART_LENGTH];
+    uint32_t counter;
+} Megolm;
+
+/**
+ * initialize the megolm ratchet. random_data should be at least
+ * MEGOLM_RATCHET_LENGTH bytes of randomness.
+ */
+void megolm_init(Megolm *megolm, uint8_t const *random_data, uint32_t counter);
+
+/** advance the ratchet by one step */
+void megolm_advance(Megolm *megolm);
+
+/**
+ * get the key data in the ratchet. The returned data is
+ * MEGOLM_RATCHET_LENGTH bytes long.
+ */
+#define megolm_get_data(megolm) ((const uint8_t *)((megolm)->data))
+
+/** advance the ratchet to a given count */
+void megolm_advance_to(Megolm *megolm, uint32_t advance_to);
+
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
+#endif /* OLM_MEGOLM_H_ */